Vibrational energy levels and predissociation lifetimes of the A2Σ+ state of SH/SD radicals by photodissociation spectroscopy

Abstract

Photo-predissociation of SH and SD radicals in the A2Σ+ state is investigated using the high- n Rydberg atom time-of-flight technique. By measuring the photoproduct translational energy distributions as a function of excitation wavelength, contributions from overlapping A2Σ+ ( v′) ← X2Π ( v″) transitions can be separated, and the H/D + S(3PJ) photofragment yield (PFY) spectra are obtained across various rovibrational levels (SH v′ = 0–7 and SD  v′ = 0–8) of the A2Σ+ ← X2Π bands. The upper A2Σ+ state vibrational levels v′ = 5–7 of SH and v′ = 3–8 of SD are determined for the first time. The PFY spectra are analyzed with the simulation program PGOPHER [C. M. Western, J. Quant. Spectrosc. Radiat. Transfer 186, 221 (2016)], which gives vibrational origins and linewidths of the rovibrational levels of the A2Σ+ state. The linewidths (≥1.5 cm−1) of the SH A2Σ+ v′ = 3–7 and SD A2Σ+ v′ = 2–8 states are characterized for the first time in this work, demonstrating that these levels undergo rapid predissociation with lifetimes on the order of picosecond. The lifetimes of the SD A2Σ+ v′ = 0, N′ = 1 and 2 levels are determined to be 247 ± 50 ns and 176 ± 60 ns by pump–probe delay measurements, respectively. The experimentally measured lifetimes are in reasonable agreement with the theoretical predictions.